Add NVIDIA Research exporter integration and local Docker quickstart

[delgadof]

Summary

Add a manual, step-by-step local OpenShell gateway → exporter → Slack quickstart, together with the receiver compatibility changes needed for the NVIDIA Research exporter. This is the full integration branch, not a documentation-only change.

• Build the latest merged source from NVIDIA/OpenShell-Research/main, using projects/openshell-exporter/ as the Docker context. No fixed exporter commit or release version; record the actual build revision in image metadata.
• Document separate gateway, Slack approval bridge, capture receiver, sandbox, and exporter setup. Use ordinary Docker bridge networking on macOS, configurable absolute paths, mTLS to the gateway, HTTPS/bearer authentication to capture, and persistent checkpoints/queues.
• Verify deployment with real sandbox network denials and a shared OCSF log volume. Approvals remain on the bridge's direct gateway connection; the exporter transports audit data.
• Support urn:openshell:event-envelope:1 CloudEvents by unwrapping the original event and preserving display context, while keeping existing bare OCSF/CloudEvents support.
• Let the capture send queue own Slack retries and log delivery acknowledgements without message contents or credentials.
• Add contract/regression tests and a separate optional synthetic developer verification helper. The quickstart itself does not inject fake audit events or approvals.
• Extend the existing sample-proposal helper with an optional sandbox token and bounded RPC deadline for trusted local demos; this is separate from the real-event quickstart.

Validation

• npm test: 211 tests passed.
• npm run typecheck and npm run build: passed.
• Quickstart/integration guide: 20 shell blocks checked with both bash -n and zsh -n.
• git diff --check: passed.
• Local demo validation during development: recreated a Ready sandbox, ran a real denied HTTPS request, observed a pending gateway proposal, and confirmed the approval request appeared in Slack. Exporter health and audit delivery were also verified.

Scope and limitations

The guide targets macOS Apple Silicon with Docker Desktop and retains its tested OpenShell gateway installation version. Only the exporter source selection follows latest main. The running demo was not rebuilt against a newer exporter revision as part of this documentation update; new exporter builds should repeat verification and review configuration/wire-contract changes. This is not a production HA, exactly-once delivery, or blanket compatibility guarantee.

No credentials, local runtime state, or exporter source code are included in this PR.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @delgadof to sign the Salesforce Inc. Contributor License Agreement.